Reads the specified number of samples from the task buffer of a digital reader task.

Namespace:  Quanser.Hardware
Assembly:  Quanser.Hardware.Hil (in Quanser.Hardware.Hil.dll)

Syntax

Visual Basic (Declaration)
Function ReadDigital ( _
	numSamples As Integer, _
	digitalBuffer As SByte() _
) As Integer
C#
int ReadDigital(
	int numSamples,
	sbyte[] digitalBuffer
)
Visual C++
int ReadDigital(
	int numSamples, 
	array<signed char>^ digitalBuffer
)
JavaScript
function readDigital(numSamples, digitalBuffer);

Parameters

numSamples
Type: System..::.Int32
The number of samples to read from the task buffer. Each "sample" consists of all the digital input channels specified when the task was created using TaskCreateDigitalReader(Int32, array<Int32>[]()[]). For example, if numSamples is 5 and the task is configured to read three channels, then the output buffer will contain 15 elements.
digitalBuffer
Type: array< System..::.SByte >[]()[]

An array for receiving the state values read from the digital inputs. The array must contain numChannels * numSamples elements, where numChannels is the number of channels specified when the task was created. The array is organized as a linear array of samples, with each sample consisting of a group of channels. For example, if digital input channels 0, 1 and 3 are being read, than the data appears in the array as follows, where the numbers correspond to channel numbers:

0 1 3 0 1 3 ...

Return Value

The return value is the number of samples read from the task buffer. This value may be less than the requested number of samples (including 0) if the task is stopped or has finished processing the total number of samples indicated in the call to Start(Hil..::.Clock, Double, Int32).

Remarks

The ReadDigital method reads the specified number of samples from the task buffer of a task created using the TaskCreateDigitalReader(Int32, array<Int32>[]()[]) method. If there's not enough samples in the task buffer, then this method will block until the requested number of samples becomes available or the task stops. Since the task reads the hardware at the sampling rate specified in the call to Start(Hil..::.Clock, Double, Int32), and stores the data in the task buffer, this method will never block for longer than the requested number of samples times the sampling period.

Because this method blocks until enough data is available and the task buffer is filled at a given sampling rate, calling this method synchronizes the caller to that sampling rate (provided the task buffer is not being filled faster than we can read the data). Thus, the ReadDigital method may be used to implement control systems, system identification, synchronous data streaming and other operations requiring a fixed sampling rate. For control systems, the numSamples parameter is typically 1, since control calculations need to be performed on each sample. For data streaming, the numSamples parameter is typically half the number of samples in the task buffer to implement double-buffering.

Warning

Many cards allow the digital I/O lines to be programmed as inputs or outputs. The digital I/O lines are configured as inputs or outputs using the SetDigitalDirections(array<Int32>[]()[], array<Int32>[]()[]) method. All the channels which will be used as digital inputs or outputs must be configured accordingly using this function. Failure to configure the digital I/O may result in the ReadDigital(Int32, array<SByte>[]()[]) method failing to read the digital I/O as expected.

Examples

This example illustrates how to read digital inputs using a task. The task reads digital channels 0-3 every millisecond using a hardware clock. The data may be processed each sampling instant. It runs for 5 seconds before stopping. Exceptions are ignored for simplicity.
C# Copy Code
int [] channels        = { 0, 1, 2, 3 };
double frequency       = 1000;
int    samples         = 5000;
int    samplesInBuffer = frequency;
int    samplesToRead   = 1;

sbyte [] buffer = new sbyte [samplesToRead * channels.Length];
Hil.Task task;

task = card.TaskCreateDigitalReader(samplesInBuffer, channels);
task.Start(Hil.Clock.Hardware0, frequency, samples);
for (int index = 0; index < samples; index += samplesToRead) {
    /* Block (if necessary) waiting for next samplesToRead samples */
    task.ReadDigital(samplesToRead, buffer);

    /* ... process sample ... */
}
task.Stop();
Visual Basic Copy Code
Dim channels() As Integer = {0, 1, 2, 3}
Dim frequency as Double = 1000
Dim samples As Integer = 5000
Dim samplesInBuffer As Integer = frequency
Dim samplesToRead As Integer = 1

Dim buffer(samplesToRead * channels.Length - 1) As SByte
Dim task As Hil.Task
Dim index As Integer

task = card.TaskCreateDigitalReader(samplesInBuffer, channels)
task.Start(Hil.Clock.Hardware0, frequency, samples)
For index = 0 To samples - 1 Step samplesToRead
    ' Block (if necessary) waiting for next samplesToRead samples
    task.ReadDigital(samplesToRead, buffer)

    ' ... process sample ...
Next
task.Stop()
Visual C++ Copy Code
array<int>^ channels        = { 0, 1, 2, 3 };
double      frequency       = 1000;
int         samples         = 5000;
int         samplesInBuffer = frequency;
int         samplesToRead   = 1;

array<char>^ buffer = gcnew array<char>(samplesToRead * channels->Length);
Hil::Task^ task;

task = card->TaskCreateDigitalReader(samplesInBuffer, channels);
task->Start(Hil::Clock::Hardware0, frequency, samples);
for (int index = 0; index < samples; index += samplesToRead) {
    /* Block (if necessary) waiting for next samplesToRead samples */
    task->ReadDigital(samplesToRead, buffer);

    /* ... process sample ... */
}
task->Stop();

Exceptions

ExceptionCondition
Quanser.Hardware..::.HilException If the read cannot be performed then an exception is thrown. This situtation typically arises if the task buffer overflowed (ran out of space) after the last call to this method.

See Also